Three forms of thin-film solar panels have been developed and commercialized in the last decade by identifying materials that are both efficient absorbers of solar power and cost-effective for manufacturer and consumer. These materials are: amorphous silicon (a-Si), cadmium telluride (CdTe) and CIGS (copper indium gallium sulfo-selenide). Although they operate effectively in thin-film (1-3 microns) form, there are both environmental and economic concerns for the cost and sustainability of the materials and processes employed in these approaches.
An alternative solution was seen in pursuing sustainable photovoltaic (PV) materials composed of Earth-abundant elements such as Cu2ZnSn(S,Se)4 (copper zinc tin sulfide-CZTS or sulfo-selenide CZTSSe) or FeS2 (iron sulfide) for the absorber layer. CZTSSe, benefiting from CIGS similarities, has already proved itself at efficiencies >12%. However, photovoltaic research using FeS2 absorbers still reports very low efficiencies (˜2%) despite this material's potential comparable to a-Si, CdS and CIGS (>20%).
The use of Fe in PV was proposed more than 25 years ago in the form of FeS2. FeS2 (also called pyrite or “fool's gold”) is an indirect band gap semiconductor with sustainable composition of abundant elements. Unfortunately, the performance problems associated with this material as a PV absorber are still not fully understood.
The appeal of FeS2, in addition to the material's low cost and abundance, has been that it exhibits a useful band gap (Eg=0.9 eV) and an absorption coefficient above 105 at Eg+0.1 eV. This high absorption coefficient makes FeS2 unique among inorganic materials, allowing downsizing the thickness of the absorber layer to lower than 0.1 μm in a solar cell able to capture most of the incident solar radiation. The attractiveness of this thickness is visible when compared to 1.5-3.0 μm for current thin-film technologies and >200 μm for single-crystal Si cells. Such thin layers not only conserve material, but they also provide an avenue to high efficiency through efficient charge separation associated with a high internal electrical field. However, the promise of FeS2 as a “golden” solution for PV has not come true to date.
Recently, a large team of scientists from NREL and Oregon State University has investigated the phenomena related to lack of performance in FeS2 and pointed to an intrinsic thermal instability of the material along with considerable challenges that must be surmounted for production of high-quality, single-phase FeS2 films. This work is reported in Yu et al., Advanced Energy Materials 1(5), 748-753 (2011). To circumvent the problem, they have used the following design principle: “select systems that do not spontaneously phase-separate into sulfur (S) deficient conducting materials with small band-gaps.” In order to provide a ligand-field splitting of sufficient magnitude for effective solar absorption the Fe2+ ion must be bound by at least six S atoms thus assuring a sufficiently large band gap. This generally requires Fe2+ in an octahedral site. Adding a third element with an electronegativity that favors strong covalent bonding with sulfur can stabilize such a site. From these considerations, this research group has chosen Fe2SiS4 and Fe2GeS4 for investigation. The analytical evaluations (summarized in Table 1, which is adapted from the aforementioned article in Advanced Energy Materials) led to the conclusion that the two materials are suitable to successfully deliver the performance originally expected from FeS2.
TABLE 1Fe2SiS4 and Fe2GeS4 evaluation resultsTGAEnthalpy ofMassCalculatedMeasuredDecom-LossCalculatedDirectDirectpositionStartingAbsorptionBandgapBandgapin BinaryPointCoefficientMaterial(eV)(eV)Sulfides (eV)(° C.)(cm−1)Fe2SiS41.551.540.591000>105Fe2GeS41.41.360.64725>105
The thermal stability of the two materials along with their close to ideal bandgap for solar cell fabrication makes the two materials good candidates for achieving the initial promise of FeS2.
To date, however, methods for preparing devices containing crystalline Fe2SiS4 and Fe2GeS4 and the like as an absorber layer using nano-scale precursors have not been available.